In monitor cameras, such as closed circuit television cameras (CCTVs) that are used for surveillance, a lens system that works effectively both during the day and at night has been desired. Particularly, outdoor monitor cameras using visible light during the day and near-infrared light at night for capturing images are useful. Therefore, lens systems with good performance in both the visible light region and the near-infrared region are desirable.
Conventional lens systems designed for visible light produce chromatic aberration in the near-infrared range. Therefore, near-infrared images captured at night are out of focus. Monitor camera lens systems need to have chromatic aberration corrected in both the visible and near-infrared light regions.
Japanese Laid-Open Patent Application 2002-207166 describes a conventional zoom lens for correcting chromatic aberrations in the visible light region and in the near-infrared region. The zoom lens described in Japanese Laid-Open Patent Application 2002-207166 is a two-group zoom lens that includes an object-side lens group having negative refractive power and an image-side lens group having positive refractive power. In one embodiment, the object-side lens group includes two lens components and three lens elements and the image-side lens group includes five lens components and six lens elements.
As described above, monitor camera lens systems should have chromatic aberration favorably corrected in both the visible light region and the near-infrared region. Other requirements for monitor camera lens systems include providing a bright image even in low illumination by reason of the lens having a low f-number, and being compact. It is desirable to develop a lens system that satisfies these requirements.